1. Field of the Invention
The present invention relates generally to flow sensors and, more particularly, to a flow sensor with an integrated flow restrictor.
2. Description of Related Technology
Flow rate control mechanisms are used in a variety of flow systems as a means for controlling the amount of fluid, gaseous or liquid, traveling through the system. In large-scale processing systems, for example, flow control may be used to affect chemical reactions by ensuring that proper feed stocks, such as catalysts and reacting agents, enter a processing unit at a desired rate of flow. Additionally, flow control mechanisms may be used to regulate flow rates in systems such as ventilators and respirators where, for example, it may be desirable to maintain a sufficient flow of breathable air or provide sufficient anesthetizing gas to a patient in preparation for surgery.
Typically, flow rate control occurs through the use of control circuitry responsive to measurements obtained from carefully placed flow sensors. One such flow sensor is a thermal anemometer with a conductive wire extending radially across a flow channel and known as a hot-wire anemometer. These anemometers are connected to constant current sources which cause the temperature of the wire to increase proportionally with an increase in current. In operation, as a fluid flows through the flow channel and, thus, past the anemometer, the wire cools due to convection effects. This cooling affects the resistance of the wire, which is measured and used to derive the flow rate of the fluid. Another form of thermal anemometer flow sensor is a microstructure sensor, either a microbridge, micro-membrane, or micro-brick, disposed at a wall of a flow channel. In this form, the sensors ostensibly measures the flow rate by sampling the fluid along the wall of the flow channel. In either application, the thermal anemometer flow sensor is disposed in the flow channel for measuring rate of flow.
There are numerous drawbacks to these and other known flow sensors. One drawback is that the proportional relationship upon which these sensors operate, i.e., that the conductive wire or element will cool linearly with increases in the flow rate of the fluid due to forced convection, does not hold at high flow velocities where the sensors become saturated. This saturation can occur over a range from 10 m/s to above 300 m/s depending on the microstructure sensor, for example. As a result, in high flow regions, measured resistance of an anemometer, or other sensor, no longer correlates to an accurate value of the flow rate. Furthermore, because these sensors reside in the main flow channel, they are susceptible to physical damage and contamination.
In addition to these drawbacks, known flow sensors are susceptible to mis-measurement due to turbulent flow effects, i.e., non-uniformity in flow velocity and pressure, both of which exist to some degree in all flow systems. Furthermore, conventional hot-wire anemometers have a slow time response and therefore do not produce accurate flow rate values upon abrupt changes in flow velocity. In addition, they require high input power to keep the entire length of wire at an elevated temperature at zero flow.
In contrast, wall-mounted thermal microstructures may have a relatively fast time response, but offer little advantage over the hot-wire anemometers because their response times are too fast, producing flow rate values that fluctuate with turbulence conditions instead of averaging out the noise associated with such turbulence. Therefore, as the flow rate of the fluid increases, turbulence increases and the wall-mounted thermal microstructure will produce increasingly erratic measurements in response thereto.
An indirect flow sensor measuring technique that measures flow rate from a sensor positioned outside of the flow channel and improves upon some of the drawbacks of the foregoing, has been designed. In one form, xcex94P pressure sensors measure a pressure drop across a flow restrictor, which acts as a diameter reducing element in the flow channel thereby creating a difference in pressure between an entrance end and an exit end of the flow restrictor. These flow restrictors have been in either honeycomb-patterned or porous metal plate flow restrictors. The pressure sensors are disposed in dead-end channels to measure the pressure drop due to the flow restrictor, with this pressure drop being proportional to the flow rate of the fluid. In other forms, the indirect flow mechanism can use a translucent tube disposed near the flow channel with a free-moving ball or indicator that rises and falls with varying flow rate conditions in the flow channel, or a rotameter, such as a small turbine or fan, that operates as would a windmill measuring wind rate.
Though they offer some improvements over sensors disposed directly in the flow channel, all of these indirect flow sensors are hampered by calibration problems. An indirect flow sensor may be calibrated to work generally with certain types of restrictors, e.g., honeycomb restrictors, but imprecise restrictor geometry results in variations in pressure and, therefore, variations in measured flow rate. And, furthermore, the sensors are not calibrated for use with other types of restrictors.
In addition to these deficiencies of indirect flow sensors, known flow restrictors further hamper flow measuring mechanisms because they do not produce uniform, laminarizing flow of the fluid. Non-uniformities in the cross-sectional area and position of the orifices in known flow restrictors result in such non-uniform flow, an example of which occurs in a honeycomb restrictor where orifices abutting the outer wall of the flow channel are truncated to conform the restrictor to circular shape of the wall. Moreover, non-linear correlations between pressure and flow rate result from this non-uniformity, especially at higher flow rates.
Therefore, to overcome the foregoing shortcomings, it is desirable to have a flow sensor and integrated flow restrictor that reduces calibration errors and that is adapted to reduce the flow rate measurement errors created by turbulence effects at the outer edges of the flow channel and to do so at an affordable cost.
In accordance with an aspect of the invention, an integrated module for measuring a flow rate of a fluid in a flow system has a housing that defines a flow channel through which a portion of the fluid flows and a flow restrictor, with an entrance end and an exit end, disposed in the flow channel. The flow restrictor comprises a plurality of orifices adapted to the flow channel to produce substantially uniform flow across the flow channel at the exit end.
In accordance with another aspect of the invention, a module for measuring a flow rate of a fluid in a flow system is provided with a flow restrictor disposed in a flow channel; a flow sensor disposed in a sensing channel communicating with the flow channel via a sensing tap with an inlet end and an outlet end such that the flow restrictor creates a pressure drop across the sensing channel allowing a portion of the fluid in the flow channel to flow into the sensing channel; and a housing wherein the flow restrictor, the flow sensor, and the sensing channel are integrally formed.
In accordance with yet another aspect of the invention, a flow restrictor, for use in a flow channel, comprises a plurality of orifices adapted to the flow channel to produce a substantially uniform flow across the flow channel at an exit end of the flow restrictor.